Negative pericardial pressure can impede ventricular systole in elasmobranch fishes

Abstract

The duration of ventricular contraction in elasmobranch fishes (sharks and rays) has been thought to greatly exceed ventricular filling, and was posited that this long systolic event is required for developing negative extracardiac (pericardial) pressure for atrial filling. Conversely, this temporal extension could be due in part to the negative force – that can be artificially induced in the pericardium ‐ impeding contraction or to an absence of key protein(s) affecting the contractile rate. This study evaluates if ventricular systole is indeed intrinsically prolonged in the absence of a negative pericardial pressure and examines whether cAMP activity, a key element of the contractile process, was attenuated in anesthetized shortfin mako (Isurus oxyrinchus) and blue (Prionace glauca) sharks. Ventricular pressures were obtained with the pericardium exposed by a pressure transducer, and ECG signals were used to discern the systolic and diastolic durations. Systolic and diastolic durations were similar for both species with systole accounting for about a third of the cardiac cycle, which differs from classical observations (Table). The activities of cAMP performed on the shark's ventricular tissues were comparable to mammals, implying that the initial phase of the contractile process was not hindered (Table). These findings suggest that the prolonged systolic event observed previously could be the manifestation of a strong negative pericardial pressure, however, similar chronic measurements on conscious sharks are needed. Data expressed as mean±sem (n). Supported by an unrestricted grant from IGAM